The invention relates to a machine for working, in particular cutting, metallic pieces using, on the one hand, a working torch, in particular a plasma cutting torch, and, on the other hand, a capacitative sensing probe used to determine and adjust the distance separating said torch from the workpiece, in particular to be cut.
Conventionally, when it is desired to carry out a cutting or welding of one or several metallic pieces by means of an automatic machine comprising a working torch, such as a welding or cutting torch, there is used a system for determining and regulating the distance separating the torch from the piece to be cut or welded.
The operation for determining and adjusting the tool/workpiece distance is now called sensing.
At present, the sensing systems used in cutting installations incorporating an automatic plasma cutting torch are detectors in the form of rings called capacitative probes.
During their operation, these systems do not require direct contact between the detector and the metallic sheet or piece to be worked on, particularly to be cut.
Thus, the manner of operation of this type of capacitative probe is based on a determination of the value of capacitance existing in the space or interval separating the capacitative probe from the sheet to be worked on.
After determination, the value of capacitance is transmitted to an electronic drive system which takes account of this value of capacitance to control a slave device permitting moving the torch and the capacitative probe fixed to the latter, which is to say the torch/capacitative probe assembly, in a plane substantially parallel to the plane of the workpiece, by maintaining a substantially constant value of capacitance and hence a distance which is also equal, between the torch and the workpiece.
In other words, a sensing device for an automatic cutting or welding machine for metal comprises, in general, a capacitative probe, conventionally in the form of a ring, fixed to the torch, and an electric connection cable permitting connection between the capacitative probe and the drive means.
However, so as to be able to maintain a substantially constant distance between the tool, which is to say the torch, and the metallic workpiece, it is necessary that the value of capacitance of the capacitative probe not be disturbed during its operation by variations of parasitic capacitances and this despite the unfavorable environment, namely particularly high temperature, generated smoke and projections of molten metal.
Thus, the value of the capacitance prevailing between the capacitative probe and the piece to be cut is only several picofarads, which is a very low value.
However, the value of the parasitic capacitance of a connecting cable is conventionally 10 to 100 times greater than that of the useful measurement, and this is true even when care is taken to use special coaxial cables, which is to say cables with very low intrinsic capacitance.
It will therefore be easily understood that, to carry out a capacitance measurement under these conditions, is not an easy matter.
Moreover, during a cutting operation, the capacitative probe frequently receives, at the moment of striking the arc, molten metal bits which can fix on the latter, thus giving rise to an untimely variation in the value of capacitance and hence an undesired movement of the torch relative to the workpiece, from which displacement can result a substantial drop in the quality of the cut, for example an undesirable adherence of slag on the edges of the fillet, an inappropriate or inadequate width and/or straightness of the cut fillet.
Moreover, the capacitative probe and its connection cable are usually also subjected to fairly large variations of temperature which give rise to a change in the characteristics of their dielectric and hence, in an analogous manner, a variation of the value of capacitance, and correspondingly of the sensing height.
The problem is therefore to be able to have a reliable and precise system for measuring the value of capacitance which does not have the above drawbacks, which is to say to be able to provide an effective sensing system which will not be affected in particular by parasitic capacitances arising from connection cables or the like.